The present invention concerns a power train of an automatic transmission for a vehicle.
Generally, an automatic transmission for a vehicle includes a transmission control unit which automatically controls a plurality of clutches and brakes mounted in a gear train so as to adjust the gear ratio of planetary gears according to the speed and load. While a power train must normally include a compound planetary gear unit and at least five friction elements in order to produce four-forward speed ratio and one-reverse speed ratio, to improve the shifting operation more effectively, a compound planetary gear unit including seven friction elements and three one-way clutches is required. This results in a complicated construction of the power train and an increase of the weight.
Further, the conventional automatic transmission comprises a limited number of shift phases, so that shifting shock impulses unavoidably occur and the speed shifting range is also restricted due to the limited number of the gear ratios, thus making it difficult to achieve the optimum ratio of fuel and power performance. Moreover, the shifting shock impulses frequently occur due to the great input torque and frequent shifting operation in the low speed section. In addition, the input shaft and the gear train must be directly connected with each other in the high speed region in order to make the power transmission efficiency better. But the conventional transmission has no such means.